More and more functions are being implemented using electrical components in modern motor vehicles. An ever higher demand for electrical power thus arises. In order to cover this demand, the efficiency of the generator system in the motor vehicle must be increased. Up to this point, PN diodes were typically used as the Z diodes in the motor vehicle generator system. Advantages of the PN diodes are, on the one hand, the low reverse current and, on the other hand, the high robustness. The main disadvantage is the high forward voltage UF. At room temperature, current does not begin to flow until UF=0.7 V.
Under normal operating conditions, e.g., a current density of 500 A/cm2, UF rises to >1 V, which means a non-negligible loss of efficiency.
Schottky diodes are theoretically available as an alternative. Schottky diodes have a significantly lower forward voltage than PN diodes, for example, 0.5 V to 0.6 V at a high current density of 500 A/cm2. In addition, Schottky diodes offer advantages during rapid switching operation as majority carrier components. The use of Schottky diodes in motor vehicle generator systems has heretofore not occurred, however. This is to be attributed to several decisive disadvantages of Schottky diodes: 1) higher reverse current in comparison to PN diodes, 2) strong dependence of the reverse current on the reverse voltage, and 3) poor robustness, in particular at high temperature. Therefore, there are ideas and concepts for improving Schottky diodes. Two examples are described below.
So-called junction barrier Schottky diodes (JBS) are described in H. Kozaka, etc., “Low leakage current Schottky barrier diode,” Proceedings of 1992 International Symposium on Power Semiconductors & ICs, Tokyo, pp. 80-85. As may be inferred from FIG. 1, a JBS includes an n+-substrate 1, an n-epitaxial layer 2, at least two p-wells 3 diffused into n-epitaxial layer 2, and metal layers on front side 4 and rear side 5 of the chip. Electrically considered, the JBS is a combination of a PN diode, i.e., a PN junction between p-wells 3 as the anode and n-epitaxial layer 2 as the cathode and a Schottky diode having the Schottky barrier between metal layer 4 as the anode and n-epitaxial layer 2 as the cathode. The metal layer on rear side 5 of the chip is used as the cathode electrode; the metal layer on front side 4 of the chip is used as the anode electrode having ohmic contact to p-wells 3 and simultaneously as the Schottky contact to n-epitaxial layer 2.
Because of the small forward voltage of the Schottky diode in comparison to the PN diode, currents only flow in the forward direction through the area of the Schottky diode. As a result, the effective area, i.e., the area per unit of area for the current flow in the forward direction, is significantly lower in a JBS than in a conventional planar Schottky diode.
In the reverse direction, the space charge regions expand with increasing voltage and collide in the middle of the area between adjacent p-wells 3 at a voltage which is lower than the breakdown voltage of the JBS. The Schottky effect, or barrier lowering effect, which is responsible for the high reverse currents, is thus partially shielded and the reverse current is reduced. This shielding effect is strongly dependent on structural parameters Xjp (penetration depth of the p-diffusion), Wn (distance between the p-wells), and Wp (width of the p-well) and of doping concentrations of p-well 3 and n-epitaxial layer 2, see FIG. 1.
P-wells 3 of a JBS may be implemented via p-implantation and subsequent p-diffusion. Through lateral diffusion in the x-direction, whose depth is comparable to the vertical diffusion in the y-direction, cylindrical p-wells result in the two-dimensional illustration, i.e., infinite length in the z-direction perpendicular to the x-y-plane, whose radius corresponds to penetration depth Xjp. Because of the radial extension of the space charge regions, this form of p-wells does not display very effective shielding of the barrier lowering effect. It is not possible to amplify the shielding effect solely through deeper p-diffusion, since the lateral diffusion correspondingly becomes wider at the same time.
Decreasing distance Wn between the p-wells is also not a good solution, since in this way the shielding effect is amplified, but the effective area for the current flow in the forward direction is reduced some more.
An alternative for improving the shielding effect of the barrier lowering effect of a JBS is the so-called trench junction barrier Schottky diode TJBS having filled trenches, which is described in German Patent Application No. DE 10 2004 053 761 A. FIG. 2 shows such a TJBS. It includes an n+-substrate 1, an n-epitaxial layer 2, at least two trenches 6, which are etched into n-epitaxial layer 2, and metal layers on front side 4 of the chip as the anode electrode and on rear side 5 of the chip as the cathode electrode. The trenches are filled up using p-doped silicon or polysilicon 7. In particular, metal layer 4 may also be made up of multiple different metal layers lying upon each other. For the sake of clarity, this is not shown in FIG. 2.
Considered electrically, the TJBS is a combination of a PN diode having a PN junction between p-doped trenches 7 as the anode and n-epitaxial layer 2 as the cathode and a Schottky diode having the Schottky barrier between metal layer 4 as the anode and n-epitaxial layer 2 as the cathode. As in a conventional JBS, currents only flow in the forward direction through the Schottky diode. Because of a lack of lateral p-diffusion, however, the effective area for current flow in the forward direction is significantly greater in the TJBS than in a conventional JBS. In the reverse direction, the space charge regions expand with increasing voltage and collide in the middle of the area between adjacent trenches 6 at a voltage which is lower than the breakdown voltage of the TJBS. As in the JBS, the barrier lowering effect which is responsible for high reverse currents is thus shielded and the reverse currents are reduced. The shielding effect is strongly dependent on structural parameters Dt (depth of the trench), Wm (distance between the trenches), and Wt (width of the trench) and of doping concentrations of p-well 7 and n-epitaxial layer 2, see FIG. 2.
The p-diffusion is omitted for implementing the trenches in the TJBS. Therefore, there is no negative effect of lateral p-diffusion as in a conventional JBS. A quasi-one-dimensional expansion of the space charge regions in the mesa area between trenches 6 may be readily implemented, since depth Dt of the trench, an important structural parameter for the shielding of the Schottky effect, no longer correlates with the effective area for current flow in the forward direction. The shielding effect of Schottky effects is therefore significantly more effective than in the JBS having diffused p-wells.
On the other hand, the TJBS offers a high robustness through its clamping function. Breakdown voltage BV_pn of the PN diode is designed in such a way that BV_pn is lower than breakdown voltage BV_schottky of the Schottky diode and the breakdown occurs on the base of the trenches. During breakdown operation, the reverse current only flows through the PN junction. Forward direction and reverse direction are therefore geometrically separated. The TJBS therefore has a similar robustness as a PN diode. As a result thereof, the TJBS is well suitable as a Z diode for use in motor vehicle generator systems.